I don't have particularly soft hair, and it's curly, but I do get waves on my canopy when the dew point is very low, and I like the softer look a lot! When it's very dry it's easier for me to experiment with curl shapes, and I've been trying a more "wavy" look lately. People keep commenting on how much my hair has grown, but it's really just not as spirally as usual.

Your post caught my attention because I've recently been using a product called Miss Jessie's Pillow Soft Curls, and the softening is incredible. Seriously, my hands don't recognize the feel of my own hair when this stuff is in it. Also, it can give me hair that's closer to waves than curls, if I rake it through my hair while stretching it out. When I sleep in it, packed in a sleep cap, I have long wavy hair in the morning! This cream might be just the kind of thing you're looking for. (You can send for free samples on Miss Jessie's website.) It seems kind of expensive but the tube is HUGE and you'd probably need only a little to perk up waves and stop frizz.

Another product that's amazingly versatile is Ouidad Climate Control gel. Some people use it when they straighten their hair, so it may be able to keep your waves from curling up on you. If they poof, you can wet your hair a little and the gel reactivates.

Also, maybe just use leave-in conditioner but don't add any creams, mousses, or gels. I think most stylers can "encourage" curl (at least they seem to with my hair), so maybe just conditioner will leave your hair free to be wavy.